The invention relates to monolithic logarithmic amplifier integrated circuits and to methods for logarithmic conversion of an input signal.
Logarithmic amplifiers have been used to provide various functions. The closest prior art is believed to be the assignee""s hybrid integrated circuit LOG100 logarithmic and log ratio amplifier, the article xe2x80x9cWhat""s All This Logarithmic Stuff, Anyhow?xe2x80x9d, by Robert A. Pease, Electronic Design, Jun. 14, 1989, pp. 111-113. Also see the text xe2x80x9cFunction Circuitsxe2x80x9d by Wong and Ott, McGraw-Hill Publishing Company, New York, 1976, page 58. Logarithmic amplifiers have been used in signal compression wherein the compressive effects of the logarithmic transfer function are useful. For example, use of the assignee""s LOG100 logarithmic amplifier connected ahead of an eight-bit analog-to-digital converter can produce equivalent 20-bit converter dynamic range.
FIG. 1 is a schematic diagram of the assignee""s above mentioned hybrid integrated circuit LOG100 logarithmic amplifier. Referring to FIG. 1, the logarithmic amplifier 1A includes a first operational amplifier 11 (also referred to as operational amplifier A1) having its (xe2x88x92) input connected to an external input terminal 14 into which an input current Iin is provided by the user. The (+) input of operational amplifier 11 is connected to ground. The output of operational amplifier 11 is connected by conductor 13 to the emitter of an NPN transistor Q1, the collector of which is connected to input terminal 14. The emitter of transistor Q1 is also connected by conductor 13 to the emitter of a matched NPN transistor Q2 having its base connected to ground and its collector connected to both an external reference current terminal 15 into which a reference current Iref is supplied by the user, and to the (xe2x88x92) input of a second operational amplifier 19 (also referred to as operational amplifier A2) having its (+) input connected to ground. The output of operational amplifier 19 is connected to an external output conductor 17 on which an output voltage Vout representative of the log ratio of Iin/Iref is produced. The base of transistor Q1 is connected to an external terminal 16. Vout is connected to one terminal of a thin film resistor R2, the other terminal of which is connected to conductor 16. A xe2x80x9ccompositexe2x80x9d temperature-dependent resistor R1 having a large positive temperature coefficient (TC) is coupled between conductor 16 and ground. Resistor R1 includes a 270 ohm thin film resister R1b connected between conductor 16 and one terminal of a 220 ohm thermistor R1a, the other terminal of which is connected to ground. Composite resister R2 may be a selectable parallel combination of thin film resisters each of which has one terminal connected to terminal 16 and another terminal connected to enable the user to set the resistance of R2.
Logarithmic amplifier 1A of FIG. 1 is implemented as a hybrid integrated circuit. The thermistor R1a is formed on a discrete chip that is bonded onto the hybrid integrated circuit. Because of its large size, the logarithmic amplifier 1 of prior art FIG. 1 must be packaged in a larger package.
FIG. 2 shows a schematic diagram of another prior art logarithmic amplifier 1B similar to that of FIG. 1 except that transistors Q1 and Q2 have been replaced by (or are represented by) diodes D1 and D2, respectively.
Generally, it is more convenient and less expensive to integrate all the elements of a circuit into a single chip. Furthermore, monolithic construction also facilitates assembly of the circuit into small surface mount packages, such as the SO-14. Accordingly, the prior art logarithmic amplifier shown in FIG. 1 has the disadvantages that the hybrid LOG100 product is not xe2x80x9ccompatible withxe2x80x9d ordinary monolithic integrated circuit (IC) technology. However, adding the capability of providing a conventional thermistor in a conventional IC process would have resulted in additional complexity and cost.
Thus, the LOG100 design shown in FIG. 1 was considered impractical to implement on a single chip, because a thermistor which could, as a practical matter, have been provided on the same chip along with the amplifier circuitry and thin film resisters, was not available. It would have been considered impractical, in view of the benefit, to add the semiconductor processing steps that would have been needed to include a thermistor in a single-chip implementation of the LOG100.
Until now no one has provided a logarithmic amplifier similar to the ones shown in FIGS. 1 and 2 integrated into a single monolithic chip and capable of being packaged in a small, inexpensive plastic package, such as a TSSOP-14 or a SO-14.
In the past, integrated circuit interconnection metallization generally has only been utilized for making very low resistance resisters. For example, very low value resisters, e.g., emitter resisters and shunt resisters having very small resistances have been formed of the integrated circuit interconnection metallization that also is used throughout the integrated circuit. U.S. Pat. No. 4,990,803 (Gilbert) issued Feb. 5, 1981 discloses a multi-stage logarithmic amplifier in which a front end PTAT resistive attenuator includes an input voltage divider circuit including a high temperature coefficient resistor and a fixed resistor in its transfer branch. The output of the attenuator is connected to a logarithmic cell circuit. U.S. Pat. No. 4,990,803 also discloses that the high temperature coefficient resistor can be a 30 ohm resistor fabricated from aluminum interconnection metallization provided during chip fabrication. An input attenuator is suitable for voltage inputs, but would shunt low level current inputs.
Thus, there has been a long-standing unmet need for a monolithic temperature-compensated logarithmic amplifier.
Accordingly, it is an object of the invention to provide a monolithic integrated circuit logarithmic amplifier and method which provide essentially temperature-compensated logarithmic amplification of an input signal.
It is another object of the invention to avoid the large physical size and high cost of prior hybrid integrated circuit logarithmic amplifiers.
It is another object of the invention to provide a small, low-cost temperature-compensated logarithmic amplifier, especially one that is suitable for measurement of light intensity in fiber-optic devices.
It is another object of the invention to avoid the difficulties of using discrete large positive-temperature-coefficient thermistors in logarithmic amplifiers.
Briefly described, and in accordance with one embodiment, the invention provides a temperature-compensated monolithic logarithmic amplifier including a logarithmic amplifier cell (26) configured to produce a logarithmic voltage signal (V3) representative of a difference between a first voltage (V1) developed across a first PN junction device (D1) in response to an input signal (Iin) and a second voltage (V2) developed across a second PN junction device (D2) in response to a reference signal (Iref). The logarithmic amplifier includes an output circuit (36) including an output amplifier (A2), a temperature-dependent first resistive element (R1) having a positive first temperature coefficient, and a second resistive element (R2) having a second temperature coefficient that is of substantially lower magnitude than the first temperature coefficient, the first (R1) and second (R2) resistive elements being coupled as a voltage divider between an output of the output amplifier (A2) and a reference conductor (GND) to provide a feedback signal to an input of the output amplifier (A2), the output circuit (36) being configured to produce a temperature-compensated output signal (Vout) in response to the logarithmic voltage signal (V3). A temperature-dependent third resistive element (R1a) included in the first resistive element (R1) is composed of conductive material which is integral with a semiconductor manufacturing process utilized to fabricate the monolithic logarithmic amplifier circuit. In one embodiment, the conductive material is aluminum or aluminum alloy interconnection metallization utilized as interconnection metallization throughout the monolithic logarithmic amplifier.